An Italian forensic police expert who investigated the Amanda Knox murder contends that evidence linking Knox and her co-defendant, Raffaele Sollecito, to the murder of Meredith Kercher was not contaminated.

Patrizia Stefanoni examined DNA during the investigation of the 2007 killing. However, a panel of experts Tuesday criticized Stefanoni’s methods, saying they were flawed and contaminated crucial evidence.

The key piece of evidence is a 30-cm kitchen knife considered to be the murder weapon, which was found in Sollecito’s apartment. Stefanoni told an appeals court that she can rule out contamination on the knife that contained DNA from both Knox and Kercher.

However, court-appointed experts said an independent review of genetic traces of Kercher are too low to be considered enough to convict Knox. More specifically, the evidence collected during the original investigation fell well below international standards.

The panel found the DNA was improperly stored in plastic bags that would degrade genetic traces, which could have been prevented by using the international standard of paper bags. Also experts said a bra clasp with DNA that placed Sollecito at the murder scene was “dirty” and had rusted.

Stefanoni defended the methods and equipment used during the investigation.

The experts concluded that due to the risk of contamination and low trace amounts of DNA, the evidence should be “inadmissible.”

In 2007, Knox and Sollecito were found guilty of murdering Kercher. The 21-year-old British exchange student had been in Italy for only two months when her body was found semi-naked with her throat slit.

Knox was sentenced to 26 years and Sollecito was ordered to serve 25 years in prison. A third defendant, Rudy Guede, was also convicted in a separate trial for participating in the murder and is serving a 16-year sentence.

Reportedly, no other physical evidence links Knox to the murder; therefore, the appellate court may decide the DNA evidence is inconclusive. The appellate trial is expected to continue through the week and a verdict is expected later in September.